The present invention relates to an electromagnetic quadrupole circuit, and is suitably applied to an electromagnetic quadrupole circuit for correcting the spot form of an electron beam in a cathode ray tube.
Referring to FIG. 7, in a conventional color CRT composed of a panel 2a, a funnel-shaped funnel part 2b, and a neck part 2c, the inside of the panel 2a is coated with three primary color stripe fluorescent material to be a fluorescent surface 3, and a color selection electrode or the like, not shown, is provided to irradiate a prescribed three primary color stripe of the fluorescent surface 3 with a three primary color beam.
An ungulate deflecting yoke 6 is provided on the outside around the border of the neck part 2c of a tube 2 and the funnel part 2b, an anode button AB is provided on the funnel part 2b to supply anode voltage, and the inside of the funnel part 2b is coated with a neck carbon CB to which the anode voltage is supplied.
An electron gun 5 is provided inside the neck part 2c of the tube 2, in which cathodes RK, GK, BK for R (red), G (green), B (blue), a first grid G1 and second grid composing a prefocus part, a third grid G3, fourth grid G4 and fifth grid G5 composing a main lens, and an electrostatic deflector 33 are sequentially arranged.
In such a CRT having the aforementioned arrangement, an electron beam 7 emitted from the three primary color cathodes RK, GK, BK toward the fluorescent surface of the panel 2a must form a circle beam spot with a small diameter on the screen of the fluorescent surface 3, however, a stigmatic distortion occurs in a beam spot due to uneven deflected magnetic field from the deflecting yoke and the like.
The diameter of the electron beam has a certain degree of deformation, and the electron beam spot is deformed by power which changes depending on a point. As a typical example, it is longitudinally long as shown in the center of FIG. 6.
In order to correct the form of the beam spot on this screen, an electromagnetic quadrupole coil is arranged on the neck of the fourth grid G4 at the center position of the main lens of the electron gun, and for example, a dynamic voltage of parabolic waveforms in synchronization with a horizontal deflection or vertical deflection, not shown, is applied to this electromagnetic quadrupole coil, thereby obtaining an even beam spot with a small diameter on the screen.
FIG. 4 shows an example of such an electromagnetic quadrupole circuit which applies a dynamic voltage in synchronization with a horizontal deflection. In this electromagnetic quadrupole circuit of FIG. 4, a first variable power source 3 is supplied to a series circuit of a diode 4 and a transistor 5 via a first choke coil 2.
In addition, a first capacitor 6 is connected in parallel to the series circuit of the diode 4 and transistor 5, a series circuit of a second capacitor 7 and electromagnetic quadrupole coil 8 is connected in parallel to the first capacitor 6, and a second variable power source 10 is supplied to the electromagnetic quadrupole coil 8 via the second choke coil 2.
A pulse in synchronization with the horizontal scanning frequency of the CRT shown in FIG. 5(A) is supplied to the base of the transistor 5 which thereby changes its state between ON and OFF.
When the transistor 5 is in an OFF state, the first capacitor 6, second capacitor 7 and electromagnetic quadrupole coil 8 forms a first resonance circuit having a high resonance frequency.
FIG. 5(B) shows a voltage waveform V1 of the first resonance circuit which is obtained at the connecting point of the first capacitor 6 and the second capacitor 7. The transistor 5 is arranged to drive before the resonance cycle finishes, the first capacitor 6 is shortened by the transistor 5 and diode 4 just when the next resonance cycle starts and the voltage exceeds the conducting voltage of the diode 4, and thereby the second capacitor 7 and electromagnetic quadrupole coil 8 form the second resonance circuit having a low resonance frequency. In this way, zero-volt switching is performed by the transistor 5 and the diode 4.
In the electromagnetic quadrupole circuit 1, the transistor 5 becomes to be in an ON or OFF state, in synchronization with the horizontal scanning frequency, thereby alternatively forming two resonance circuits having different resonance frequencies. As a result, the driving current i of a bathtub waveform in synchronization with the horizontal deflection as shown in FIG. 5(C) is supplied to the electromagnetic quadrupole coil 8.
FIG. 6 shows such an electromagnetic quadrupole coil 8 attached to the neck part 2c of the CRT, in which a coil 8C is wound onto each end of two yokes 8A and 8B arranged opposite each other. The spot of the electron beam is deformed by the magnetic field F which occurs by the quadrupole coil driving current i flowing along the coil 8C, so as to dynamically correct the focus.
In the electromagnetic quadrupole circuit 1 having such an arrangement, the first variable power source 3 mainly sets the alternating current of the quadrupole coil driving current i, that is, a peakxe2x80xa2peak value ipp, and the second variable power source 10 controls the value of current which flows around the electromagnetic quadrupole coil 8 via the second coil 9, in order to mainly set the direct current of the quadrupole coil driving current i, that is, a bottom value ib.
These peakxe2x80xa2peak value ipp and bottom value ib, however, can not be controlled independently. That is, when the voltage of the first variable power source 3 is changed to change the peakxe2x80xa2peak value ipp, the bottom value ib is changed accordingly. In addition, when the voltage of the second variable power source 10 is changed to change the bottom value ib, the peakxe2x80xa2peak value ipp is changed accordingly. This is the reason why the peakxe2x80xa2peak value ipp and bottom value ib are difficult to set to be optimal values, which causes a problem in that the optimal correction of the spot form is difficult.
Further, in the electromagnetic quadrupole circuit 1 having such an arrangement, the value of the quadrupole coil driving current i is changed in accordance with the different features of the parts composing the circuit and the changes in features due to temperature, which causes a problem in that the optimal correction of the spot form is difficult.
The present invention has been made considering the above points, and intends to propose a simply constructed electromagnetic quadrupole circuit capable of reliably correcting the spot into an optimal form.
To solve the problems, the following are employed in the present invention: a switching means which becomes to be in an ON state during the horizontal scanning of a horizontal synchronization signal, the switching means having one control terminal connected to the high electric potential point and the other control terminal connected to the low electric potential point, and the control terminals receiving a driving signal in synchronization with the horizontal synchronization signal; a series circuit which resonates with a first resonance frequency during the horizontal scanning, the series circuit arranged in parallel to the two control terminals of the switching means, including a first resonance capacitor and an electromagnetic quadrupole coil connected in series; a second resonance capacitor which resonates with a second resonance frequency together with the first resonance capacitor and the electromagnetic quadrupole coil when the switching means in an OFF state, the second resonance capacitor arranged in parallel to the series circuit; a current detecting means for detecting the alternating current and direct current of current flowing along the electromagnetic quadrupole coil; a control means for supplying a prescribed alternating current control value and direct current control value; a first power supply means for supplying first voltage based on the alternating current detected by the current detecting means and the alternating current control value outputted from the control means, the first power supply means connected to one control terminal of the switching means; and a second power supply means for supplying second voltage based on the direct current detected by the current detecting means and the direct current control value outputted from the control means, the second power supply means connected to the connecting point of the first resonance capacitor and electromagnetic queadrupole coil of the series circuit.
The control means controls the alternating current control value and direct current control value so that the alternating current and direct current of current flowing along the electromagnetic quadrupole coil have prescribed values.
The alternating current and direct current of current flowing along the electromagnetic quadrupole coil are detected, the feedback of the amount of supply current from the first power source is controlled based on the alternating current and alternating current control value, and the feedback of the amount of supply current from the second power source is controlled based on the direct current and direct current control value, so that the driving current of the electromagnetic quadrupole coil can be controlled to have an optimal value, regardless of different features of parts composing the circuit and the change in features due to temperature, thus making it possible to always perform the optimal correction of the spot form.